Spread spectrum handshake for digital subscriber line telecommunications systems

ABSTRACT

Handshake information for xDSL services is transmitted utilizing a spread spectrum modulated system where a plurality (n) of carrier tones (n&gt;2) are summed and utilized as a spread spectrum carrier (SSC), and data is modulated onto the carrier (at all utilized frequencies). Preferably, phase shift keying (PSK) modulation or a variation thereof is used as the encoding/modulation technique.

This Application is a 371 of PCT/US9913817, filed on Jun. 18, 1999,which claims benefit of provisional application Serial No. 60/090,333filed Jun. 23, 1998 which is hereby incorporated by reference in itsentirety herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates broadly to telecommunications systems andmethods. More particularly, the present invention relates to a handshakefor an xDSL (Digital Subscriber Line type) modem.

2. State of the Art

Digital subscriber line (DSL) systems are a new and fast-growing datatransmission service which provide significantly higher data rates thanconventional V.34 and V.90 type modems. The abbreviation “xDSL” is anintegrated designation for different DSL services including ADSL(asymmetric DSL), SDSL (symmetric DSL), RADSL (rate-adaptive DSL), HDSL(high speed DSL), and VDSL (very high speed DSL), UDSL (universal DSL),and their modifications such as ADSL-LITE (also known as G.lite). ThexDSL services typically provide data rates of several Mbits/s downstreamand several hundred Kbits/s upstream, although SDSL provides the sameupstream and downstream rates. All types of DSL are based on discretemultitone (DMT) technology although they have different parameters. See,J. Makris, “DSL Services”, Data Communications, April 1998, and ANSIT1.413-1995 “Network and Customer Installation Interfaces—AsymmetricalDigital Subscriber Line (ADSL) Metallic Interface”.

According to the ITU-T telecommunications standards for the xDSLservices, at modem start-up a handshake procedure (called G.hs) isutilized. The requirements for G.hs are set forth in several documentssuch as “Proposal for G.hs Modulation Technique and Message Protocol”,TTU-T Telecommunication Standardization Sector, C1-068 Chicago, USA 6-9Apr. 1998, and “Handshake procedures for Digital Subscriber Line (DSL)transceivers”, ITU-T Draft G.994.1 (Feb. 3, 1999) which are both herebyincorporated by reference herein in their entireties. The mainrequirements of the handshake are: transmission of several tens of bytesduring the handshake; signal compatibility with all types of DSLreceivers; and interworking with the plain old telephone service (POTS),the integrated services digital network (ISDN), and time compressionmultiplexing ISDN (TCM-ISDN). Meeting these main requirements is not atrivial task because of considerable noise and cross-talk impairments,and lack of knowledge regarding the frequency characteristics of thechannel, all of which is described in various papers such as: MatsushitaElectric Industrial Co. Ltd, “Proposed Working Text for G.hs Based onV.8bis”, ITU-Telecommunication Standardization Sector, NF-044, Nice,France, 11-14 May 1998; Matsushita Electric Industrial Co. Ltd.,“Spectrum Considerations for G.hs”, TTU-TelecommunicationsStandardization Sector, NF-045, Nice, France 11-14 May 1998; MatsushitaElectric Industrial Co., Ltd., “Crosstalk Model Proposed Working Textfor G.hs Test” ITU-Telecommunications Standardization Sector, NF-046,Nice, France 11-14 May 1998; NEC, “Desired Spectrum Range for G.hs underTCM-ISDN”, ITU-Telecommunications Standardization Sector, NF-066, Nice,France 11-14 May 1998; and 3Com, “Proposed Spectrum and Tone Selectionfor G.hs”, ITU-Telecommunications Standardization Sector, NF-068, Nice,France 11-14 May 1998.

More particularly, signal attenuation across lines carrying xDSL signalsis a non-monotonic function of frequency, and may have several deepnotches, while noise power spectral density (PSD) is also not a flatfunction of frequency. As a result, the signal to noise ratio (SNR) is acomplex multiextremes function of frequency. Moreover, the SNR issubject to random and cyclic variations in time. For example, in theTCM-ISDN environment which includes the so-called “ping-pong mode” ofup-and down-transmissions, far-end cross-talk (FEXT) and near-endcross-talk (NEXT) interleave at a frequency of 400 Hz. Since FEXT andNEXT processes have significantly different power spectral densities,significant NEXT noise is introduced every other 1.25 milliseconds.

As set forth above, several authors have made proposals regarding G.hstechniques. The core of these proposals has been two-tone transmissionwith different bit rates depending upon the noise environment. Frequencydiversity is provided by bits duplication on nominal and backup carriertones. Time diversity is provided by increasing the symbol interval(i.e., decreasing the symbol rate). These proposals have severaldisadvantages. First, both the nominal and backup tones may be locatedin notches or other frequency domain areas having a low SNR, thusrendering the handshake ineffective. Second, increasing the symbolinterval may not be sufficient to account for bursty noise. For example,in the TCM-ISDN environment, the signal to noise ratio may be below anacceptable level every other 1.25 ms interval. Even if the initialsymbol interval of 0.232 ms were to be increased by a factor of four to0.928 ms as suggested by one of the authors in the art, the entireinterval could be located within the 1.25 ms high noise window. In fact,even increasing the symbol interval by a factor of 8 would still onlyprovide a final symbol interval of 1.885 ms which could be 67% coveredby the low SNR area.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a handshake for anxDSL modem which meets proposed xDSL standards requirements.

It is another object of the invention to provide a handshake for an xDSLmodem which has excellent frequency diversity and time diversity andprovides excellent reliability.

It is a further object of the invention to provide an xDSL modemhandshake which utilizes multitone signaling.

It is an additional object of the invention to provide an xDSL modemhandshake which will interwork with existing telecommunicationsservices.

Another object of the invention is to provide modems and methods forimplementing the above-listed objects.

In accord with the objects of the invention, handshake information forxDSL services are transmitted utilizing a spread spectrum modulatedsystem where a plurality (n) of carrier tones (n>2) are summed andutilized as a spread spectrum carrier (SSC), and data is modulated ontothe carrier (at all utilized frequencies). Preferably, phase shiftkeying (PSK) modulation (or a variation thereof such as BPSK—binary PSK,or DBPSK—differential binary PSK) is used as the modulating technique.When the spread spectrum carrier is modulated by handshake bitsaccording to BPSK, the SSC is transmitted with sign “+” if the handshakebit is a +1 and with sign “−” if the handshake bit is a “−1”. When usingDPSK, the same modulation procedure is used for differentially encodedhandshake bits.

According to one preferred aspect of the invention, the handshake symbolrate (SR) is set equal to 0.8A symbols/msec, where A is a positiveinteger. In order to improve reliability, symbols are preferablyrepeated at least four times. According to another preferred aspect ofthe invention, a preamble can be provided for timing recovery purposes.Further aspects of the invention include different receiver systems,including a quasicoherent receiver, an autocorrelation receiver, and apresently preferred incoherent receiver which utilizes coherentaccumulation of FFT components for a DBPSK spread spectrum handshakesignal.

Additional objects and advantages of the invention will become apparentto those skilled in the art upon reference to the detailed descriptiontaken in conjunction with the provided figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the preferred transmitter of the invention.

FIG. 2 is a diagram showing the signal structure of the preferredhandshake signal of the invention.

FIG. 3a is a block diagram of an autocorrelation receiver of DBPSKsignals according to the invention;

FIG. 3b is a block diagram of a quasicoherent receiver of DBPSK signalsaccording to the invention; and

FIG. 3c is a block diagram of an incoherent receiver which utilizescoherent accumulation of FFT components for a DBPSK spread spectrumhandshake signal according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

According to the invention, handshake information for xDSL services istransmitted by modulating the handshake information on a spread spectrumcarrier (SSC), where the SSC is a sum of tones conventionally used byxDSL for the data transmission mode. As seen in FIG. 1, the transmitter10 includes a phase initialization (PI) unit 15, an inverse fast Fouriertransformation (IFFT) unit 20, a spread spectrum carrier (SSC) memory25, a modulator 30, a differential encoder 35 and a block frame unit 40.In essence, the phase initialization unit 15 generates complex numbersindicating a desirable amplitude and initial phase distribution for aplurality of multitone signals. Preferably, the amplitude distributionis chosen to be flat (uniform). According to the preferred embodiment,the initial phases of different tones are generated randomly or selectedspecifically in order to minimize the crest-factor of the generatedtones. Regardless, where DMT-style implementation is utilized, the IFFTtransforms a set of complex numbers into a set of time-domain sampleswhich are stored in memory 25. If, for example, all or substantially alltwo hundred fifty-six DMT tones (such as might be utilized in ITU-TStandard G.992.2) are generated by the PI unit 15, a five hundred twelvesample set may be stored in the memory 25. Additional repetitive samplesmay also be stored in the memory, if desired as a prefix which can beused by the receiver to reduce distortion. If desired, the samples maybe generated in other manners (e.g., without the PI and IFFT, or inanother apparatus) and loaded and stored in the transmitter memory foruse as described below.

While all two hundred fifty-six DMT tones may be included in the spreadspectrum carrier, it should be appreciated by those skilled in the artthat according to the invention, different numbers of tones (anddifferent tones) can be used in different circumstances, provided aspread spectrum carrier is utilized. Thus, for purposes of thisapplication, a carrier may be considered a spread spectrum carrier ifthree or more distinct tones are modulated together. Thus, the SSC for adown-stream connection may contain a full or partial set of down-streamtones, while the SSC for an up-stream connection could contain a full orpartial set of up-stream tones. For example, a G.Lite ADSL up-stream SSCmight utilize allowed tones from the set six through thirty-two (25.875kHz . . . 138 KHz), while the downstream SSC might utilize allowed tonesfrom the set thirty-three through one hundred twenty-eight (142.3125 kHz. . . 552 kHz). The SSC may contain only even or odd tones to reduce theprocessing at the receiver.

Handshake information (as described below with reference to FIG. 2)which is to be modulated onto the spread spectrum carrier is provided tothe differential encoder 35 and differentially encoded bits are writtento the block frame unit 40. According to the preferred embodiment of theinvention, the handshake information is provided to the differentialencoder at a speed of 0.8 kbps, and differentially encoded 4-bitsubblocks are written into registers of the block frame unit 40.Preferably, each 4-bit subblock is read four times such that each blockframe is provided to the modulator 30 with a speed of 3.2 kbps.

When no differential encoder is utilized, the modulation technique ispreferably is a binary phase shift keying (BPSK). When a differentialencoder is utilized, the modulation technique is preferably adifferential BPSK. Regardless, the modulator 30 uses the output of theblock framer unit 40 to select whether the samples stored in the memory25 are to be transmitted as is, or inverted (i.e., multiplied by −1 or180 degrees out of phase). The samples stored in the memory 25 aresequentially read out of the memory so that all samples are modulated(i.e., transmitted as is or inverted) at the proposed symbol ratediscussed below. When BPSK is utilized, the SSC samples are transmittedwith sign “+” if the handshake bit is a “+1”, and transmitted with sign“−” if the handshake bit is “−1” (or vice versa). When using DBPSK, thesame modulation procedure is used for differentially encoded handshakebits.

It will be appreciated by those skilled in the art that while BPSK orDBPSK modulation is preferred, other modulation techniques such as QPSK(quadrature PSK), DQPSK (differential QPSK), frequency modulation,amplitude modulation, and quadrature amplitude modulation could beutilized.

Details of the handshake which modulates the SSC is seen in FIG. 2.According to the preferred embodiment of the invention, the handshakeincludes a preamble and a G.hs message. The preamble comprises Nsubblocks of a distinct four bit sequence “1,1,1,−1” followed by foursubblocks of a four bit divider sequence “−1,−1,−1,−”, followed by eightsubblocks of a pseudorandom sequence (as specified). Each subblock ispreferably generated at a 1.25 millisecond rate (i.e., each subblock hasa duration of 1.25 ms), with bits being generated at a 0.3125millisecond rate. After the preamble, the G.hs message is provided andpreferably includes N blocks which are generated at a 5 millisecondrate. Each block preferably includes four subblocks of four informationbits (symbols) each (b1, b2, b3, b4), with the four information bitsbeing repeated four times (i.e., each subblock within the block containsthe same material). Each symbol carries one information bit. So eachblock of duration 5 milliseconds, carries four information bits withredundancy 3/4. As indicated in FIG. 2, each bit of the preamble andG.hs message is preferably modulated onto a spread spectrum carrier. Asdiscussed in more detail below, the preamble is preferably provided topermit the receiver to detect G.hs transmission, to recover the spreadspectrum carrier for coherent processing, and for symbol and blocksynchronization (timing recovery). While the preamble is preferablymodulated, an unmodulated preamble (all +1s) can be utilized.

According to the preferred embodiment of the invention, a symbol rate(SR) is set equal to 0.8 symbols/millisecond, where A=1,2,3 . . . . Withthe symbol rate set in this manner, an integer number of symbols will beplaced within the 1.25 millisecond burst duration in the TCM-ISDN crosstalk environment. Thus, when A=4 (bit rate=3200 bps), half a byte (fourbits) will be transmitted within the 1.25 ms burst. When A=8 (bitrate=6400 bps), one byte will be transmitted within the 1.25 ms burst.By transmitting each symbol of the G.hs message at least four times, atleast two symbol time-separated blocks will occur within the 1.25 mshigh SNR FEXT areas in a TCM-ISDN cross-talk environment.

Taking into account the 400 Hz periodicity of the NEXT and FEXTcross-talk in TCM-ISDN systems, a noiseless time window may be found bycalculating the correlation between N-symbol blocks delayed by 2.5 msrelative to each other. If the delayed blocks coincide with each other(i.e., they have not been corrupted by noise), the time window has a“high enough” SNR (i.e., it is “noiseless” for the purpose of thehandshake) and can be used for receiving the handshake message. Thestructure of the preamble is particularly arranged to permit thisdetermination.

Because the noiseless time window has a random time position relative tothe transmission of the preamble and handshake message, receivedN-symbol blocks may be cyclically shifted. In other words, the blockframe may not correspond to the noiseless time window frame. It istherefore preferred that this shift be estimated and eliminated.According to the preferred embodiment of the invention, the cyclic shiftmay be estimated and eliminated by transmitting an N-symbol referenceblock. Thus, the preamble is provided with a series of reference blockshaving the form “1,1,1,−1”. It should be appreciated that any shift ofthe reference block will be distinct (−1,1,1,1; 1,−,1,1; 1,1,−1,1) anddetectable, and may therefore be detected and eliminated at thereceiver. This pattern therefore allows for symbol synchronization andsubblock synchronization.

Turning now to FIGS. 3a-3 c, three different receivers are shown forreceiving and demodulating the handshake signal of the invention. Anautocorrelation receiver 100 a for DBPSK spread spectrum handshakesignals is seen in FIG. 3a. The autocorrelation receiver 100 a includesan autocorrelation demodulator 102 a, a timing signal extractor 103 a,and preferably further includes a noiseless time window (TW)determination unit 104 a and a transmitted bit selection (BS) unit 106a. The autocorrelation demodulator 102 a includes a delay line (DL) 110a, a multiplier 112 a, a low pass filter (LPF) 114 a, and a binaryslicer (Sgn) 116 a. Incoming SSC modulated signals are provided to thedelay line 110 a and the multiplier 112 a. The delay Δt of the delayline is preferably set equal to 1/0.8A ms (i.e., the handshake symbolduration). Thus, the multiplier 112 a multiplies the incoming signalwith the delayed signal. The output is forwarded to the low pass filter114 a which is preferably provided with a frequency bandwidth Δfapproximately equal to A/1.25 kHz. For example, when using block lengthA=4, Δt=0.3125 ms, and Δf=3.2 kHz. The output of the low pass filter 114a reflects the modulation function in the transmitter, and the signfunction of the low pass filter output, as generated by the binaryslicer 116 a which compares the output to a zero threshold, correspondsto the transmitted bits.

As will be appreciated by those skilled in the art, the autocorrelationreceiver 100 a calculates (at the multiplier 112 a) a scaler product(S_(n)(t)*S_(n−1)(t)) between a given spread spectrum signal S_(n)(t)and a previous spread spectrum signal S_(n−1)(t). The binary symbolI_(n) received with the n-th symbol interval is therefore determinedaccording to I_(n)=sgn(S_(n)(t)*S_(n−1)(t).

As seen in FIG. 3a, the binary slicer 116 a requires timing informationwhich is preferably extracted from the low pass filter output bybandpass filtering of a frequency component responding to the baud(symbol) frequency. Alternatively (and also as shown in FIG. 3a, thetiming information can be extracted from the incoming signal by avariety of well-known methods; e.g., as taught in Jan W. M. Bergmans,Digital Baseband Transmission and Recording, Chapters 9 and 10, “Basicsof Timing Recovery”, and “A Catalog of Timing Recovery Schemes”, KluwerAcademic Publishers, Boston (1996) pp. 451-587.

While the autocorrelation demodulator 102 a in conjunction with thetiming extractor 103 a suffices as a G.hs receiver in situations whichdo not require carrier recovery or other special synchronization,additional circuitry can be utilized if desired. Thus, if the channelnoise has a steady power spectral density, robustness can be increasedby accumulating signals at the output of the low pass filter, takinginto account that every symbol may be repeated several times. Inaddition, if the PSD is known, the spread spectrum signal may be passedthrough a corresponding filter (not shown) at the input of the receiverin order to emphasize components of the spread spectrum signal having ahigher SNR.

In addition, and according to the preferred embodiment of the invention,where a preamble is utilized, a noiseless time window determination unit104 a can be provided to compare the signal subblocks containing Nsymbols and delayed relative to each other by 2.5 ms. If the delayed Nbit combination coincides within a certain time window, it indicatesthat this window has a sufficiently high SNR and can be used forreceiving window has a sufficiently high SNR and can be used forreceiving handshake bits. Regardless, the window determination unit 104finds the time window of interest and generates an output signalindicating the time position of the desired window which is provided tothe bit selection unit 106 a. The demodulated bits provided at theoutput of the slicer during the noiseless window are also provided tothe bit selection unit 106 a, which determines from the bits and thewindow information the cyclic shift in effect. Thus, during receipt ofthe G.hs message, the bit selection unit 106 a selects the correctportion of the received bits and eliminates the cyclic shift in thereceived information blocks. The bit selection unit 106 a produces foroutput N bits every 5 milliseconds.

Turning to FIG. 3b, a quasicoherent receiver 100 b for DBPSK spreadspectrum handshake signals is shown. The quasicoherent receiver 100 bincludes an autocorrelation demodulator 102 b, a timing signal extractor103 b, and preferably further includes a noiseless time windowdetermination unit 104 b and a transmitted bit selection unit 106 b. Thequasicoherent demodulator 102 b includes a spread spectrum recovery(SSCR) unit 111 b, a multiplier 112 b, a low pass filter 114 b, a binaryslicer 116 b, a delay line 118 b, and a sign multiplier 120 b. IncomingSSC modulated signals are provided to the spread spectrum carrierrecovery unit 111 b and the multiplier 112 b. The spread spectrumcarrier recovery unit 111 b accumulates SSC samples during the preambleand extracts a spread spectrum reference signal R(t) therefrom. Themultiplier 112 b multiplies the incoming signal with the output of theSSC recovery unit. The output is forwarded to the low pass filter 114 bwhich is preferably provided with a frequency bandwidth Δf approximatelyequal to N/1.25 kHz. The output of the low pass filter 114 b is fed toslicer 116 b which compares the output to a threshold (typically zero).The output of slicer 116 b is a binary signal which is fed to the delayline 118 b and to the sign multiplier 120 b. The sign of the output ofthe sign multiplier 120 b corresponds to the transmitted bits.

As will be appreciated by those skilled in the art, in the quasicoherentreceiver 100 b, the average unmodulated SSC, preferably extracted fromthe preamble by the SSC recovery unit 111 b, is used as a spreadspectrum reference signal R(t) for the coherent demodulation. Thus, therecovered binary symbol I_(n)=J_(n)*J_(n−1), whereJ_(n)=sgn(S_(n)(t)*R(t)), and J_(n−1)=sgn(S_(n−1)(t)*R(t)). Thequasicoherent receiver 100 b provides excellent results, but issubstantially more complicated to implement than the autocorrelationreceiver 100 a because of the SSC recovery unit 111 b.

The functioning of the timing signal extractor 103 b, and the timewindow determination unit 104 b and bit selection unit 106 b of thequasicoherent receiver 100 b are substantially as described above withrespect to corresponding elements of FIG. 3a.

Turning to FIG. 3c, an incoherent receiver 100 c for DBPSK spreadspectrum handshake signals is shown. As seen in FIG. 3c, the incoherentreceiver includes a fast Fourier transform block 130, a quadraturecomponent accumulation unit 135, a multichannel incoherent demodulator140, a DMT accumulation unit 145, and a binary slicer 150. The FFT block130 receives the time domain handshake signal and converts the signalinto a frequency domain signal. The output of the FFT block are signalsF_(cnkm) and F_(snkm) which are respectively, the real and complex partsfor the k-th DMT tone at the m-th DMT symbol interval of the n-thhandshake symbol. The quadrature component accumulation (QCA) unit 145separately sums the real parts together and the imaginary parts togetheraccording to$F_{cnk} = {{\sum\limits_{m}{F_{cnkm}\quad {and}\quad F_{snk}}} = {\sum\limits_{m}{F_{snkm}.}}}$

The outputs of the quadrature component accumulation unit 145 are thendemodulated by the incoherent demodulator 140 according toF_(nk)=F_(cnk)*F_(c(n−1)k)+F_(snk)*F_(s(n−1)k). The outputs of theincoherent demodulator 140 are then summed over all tones k by the DMTaccumulator (DMTA) 145 according to$F_{n} = {\sum\limits_{k}{F_{nk}.}}$

Finally, the output of the DMT accumulator 145 is provided to the binaryslicer 150 in order to compare the output F_(n) to a zero threshold. Thedecoded binary symbol I_(n)=sgn(F_(n)).

It should be appreciated by those skilled in the art that the incoherentreceiver 100 c is relatively simple to implement because it is based onthe use of a FFT which is already available in DMT-based systems. Inaddition, no frequency equalization (carrier phase recovery) isrequired, and the performance of the incoherent receiver 100 c is nearlyas good as the quasicoherent receiver 100 b of FIG. 3b.

There have been described and illustrated herein methods and apparatusfor implementing a spread spectrum handshake for a digital subscriberline telecommunications system. While particular embodiments of theinvention have been described, it is not intended that the invention belimited thereto, as it is intended that the invention be as broad inscope as the art will allow and that the specification be read likewise.Thus, while a particular transmitter and particular receivers have beendisclosed, it will be appreciated that other transmitters and receiverscould be utilized, provided that the transmitter modulate a handshakesignal onto a spread spectrum carrier. Thus the implementation of thetransmitters and receivers will partially depend upon the encodingtechnique utilized (e.g., DPSK, QPSK, etc.), the results desired, andlimitations or requirements of standards which might be applicable.Implementation of functions may also be accomplished in several manners.Thus, while slicers have been described for purposes of generatingdecoded binary signals, other apparatus well-known in the art could beutilized. Also, while a handshake sequence including a preamble and ahandshake message have been described, it will be appreciated thatdifferent preambles and different handshake messages could be provided,and/or that a handshake sequence could be provided with no preamble. Itwill therefore be appreciated by those skilled in the art that yet othermodifications could be made to the provided invention without deviatingfrom its spirit and scope as so claimed.

We claim:
 1. A digital subscriber line (DSL) type modem, comprising: atransmitter having a handshake generator which generates handshakesignals, a spread spectrum carrier generator which generates a spreadspectrum carrier including at least three tones associated with DSL typemodems, and a modulator coupled to said handshake generator and to saidspread spectrum carrier generator, said modulator modulating indicationsof said handshake signals onto indications of said spread spectrumcarrier simultaneously.
 2. A modem according to claim 1, wherein: saidmodulator modulates said indications of said spread spectrum carrieraccording to one of a phase shift keying (PSK) technique, frequencymodulation, amplitude modulation, and quadrature amplitude modulation.3. A modem according to claim 2, wherein: said PSK technique comprisesone of binary PSK, differential binary PSK, quadrature PSK, anddifferential quadrature PSK.
 4. A modem according to claim 1, wherein:said modulator modulates said indications of said spread spectrumcarrier according to differential binary phase shift keying.
 5. A modemaccording to claim 4, wherein: said spread spectrum carrier generatorcomprises memory which stores said indications of all said tones.
 6. Amodem according to claim 5, wherein: said indications comprise inversefast Fourier transform (IFFT) samples of said at least three tones.
 7. Amodem according to claim 6, wherein: said indications comprise IFFTsamples of substantially all two hundred fifty-six DMT tones associatedwith DSL type modems.
 8. A modem according to claim 1, wherein: saidspread spectrum carrier generator comprises memory which stores saidindications of all said tones.
 9. A modem according to claim 8, wherein:said indications of all said tones comprise inverse fast Fouriertransform (IFFT) samples of said at least three tones.
 10. A modemaccording to claim 9, wherein: said indications of all said tonescomprise IFFT samples of substantially all two hundred fifty-six DMTtones associates with DSL type modems.
 11. A modem according to claim 1,wherein: said handshake generator comprises a differential encodercoupled to a block framer.
 12. A modem according to claim 1, wherein:said handshake signals comprise a handshake message.
 13. A modemaccording to claim 12, wherein: said handshake message includes aplurality of blocks, each block having a plurality of repeatingsubblocks.
 14. A modem according to claim 13, wherein: said blocks havea 5 millisecond rate.
 15. A modem according to claim 14, wherein: saidsubblocks have a 1.25 millisecond rate, and each subblock contains fourbits.
 16. A modem according to claim 12, wherein: said handshake signalsfurther comprise a preamble.
 17. A modem according to claim 16, wherein:said preamble comprises a plurality of repeating subblocks.
 18. A modemaccording to claim 17, wherein: each said subblock has a 1.25millisecond rate and includes four predetermined bits, said fourpredetermined bits selected to permit a shift in phase of said fourpredetermined bits to be detected.
 19. A modem according to claim 17,wherein: said preamble further includes at least one subblock having adivider sequence, and a plurality of subblocks representing apseudorandom sequence.
 20. A modem according to claim 1, furthercomprising: a receiver having a demodulator.
 21. A modem according toclaim 20, wherein: said receiver is chosen from a group consisting of anautocorrelation receiver, a quasicoherent receiver, and an incoherentreceiver.
 22. A modem according to claim 21, wherein: said receiver isan autocorrelation receiver including a delay line which receives anddelays a received handshake signal, a multiplier which multiplies saidreceived handshake signal with an output of said delay line, a low passfilter which filters an output of the multiplier, and means forobtaining binary symbol indication from an output of said low passfilter.
 23. A modem according to claim 21, wherein: said receiver is aquasicoherent receiver including a spread spectrum carrier recovery unitgenerates a reference spread spectrum signal from a received signal, amultiplier which multiplies the received signal with said referencesignal, a low pass filter which filters an output of the multiplier, andmeans for obtaining a binary symbol indication from an output of saidlow pass filter.
 24. A modem according to claim 23, wherein: said meansfor obtaining a binary symbol indication comprises a slicer coupled toan output of said low pass filter, a delay line which receives anddelays outputs of said slicer, and a second multiplier which receives anoutput of said slicer and an output of said delay line and generatessaid binary symbol indication therefrom.
 25. A modem according to claim21, wherein: said receiver is an incoherent receiver including a fastFourier transformer (FFT) which receives an incoming time domainhandshake signal and generates real and imaginary frequency domainsignals therefrom, a quadrature component accumulation (QCA) unitcoupled to said FFT which separately sums said real frequency domainsignals together and said imaginary frequency domain signals together,an incoherent demodulator coupled to said QCA unit which combines saidsummed real and imaginary frequency domain signals, a discrete multitoneaccumulator (DMTA) coupled to said QCA unit which sums outputs of saidQCA unit over said at least three tones, and means for generating adecoded binary symbol from an output of said DMTA.
 26. A modem accordingto claim 20, wherein: said handshake signals comprise a handshakemessage and a preamble, said preamble comprises a plurality of repeatingsubblocks, wherein said receiver includes means for utilizing saidrepeating subblocks to find a high-signal-to-noise time window.
 27. Amodem according to claim 26, wherein said means for utilizing saidrepeating subblocks includes means for correlation of said repeatingsubblocks delayed relative to each other by a predetermined timeinterval.
 28. A method of transmitting digital subscriber line (DSL)type modem handshake information, comprising: generating handshakesignals; and modulating indications of said handshake signals onto aspread spectrum carrier, said spread spectrum carrier including at leastthree tones associated with DSL type modems, wherein said modulatingcomprises modulating said indications of said handshake signals ontoindications of said at least three tones simultaneously.
 29. A methodaccording to claim 28, wherein: said handshake signal indications aremodulated onto said spread spectrum carrier according to one of a phaseshift keying (PSK) technique, frequency modulation, amplitudemodulation, and quadrature amplitude modulation.
 30. A method accordingto claim 29, wherein: said PSK technique comprises one of binary PSK,differential binary PSK, quadrature PSK, and differential quadraturePSK.
 31. A method according to claim 28, wherein: said handshake signalindications are modulated onto said spread spectrum carrier according todifferential binary phase shift keying.
 32. A method according to claim28, further comprising: generating said indications by taking an inversefast Fourier transform (IFFT) of said at least three tones; and storingsaid indications in memory, wherein said modulating comprises readingsaid indications from memory in order to modulate said indications ofsaid handshake signals onto said indications stored in memory.
 33. Amethod according to claim 28, wherein: said handshake signals comprise ahandshake message, said handshake message including a plurality ofblocks, each block having a plurality of repeating subblocks.
 34. Amethod according to claim 33, wherein: said blocks have a 5 millisecondrate, said subblocks have a 1.25 millisecond rate, and each subblockcontains four bits.
 35. A method according to claim 33, wherein: saidhandshake signals further comprise a preamble.
 36. A method according toclaim 33, wherein: said preamble comprises a plurality of repeatingsubblocks, each said subblock has a 1.25 millisecond rate and includesfour predetermined bits, said four predetermined bits selected to permita shift in phase of said four predetermined bits to be detected.
 37. Amethod according to claim 36, wherein: said preamble further includes atleast one subblock having a divider sequence, and a plurality ofsubblocks representing a pseudorandom sequence.